They divide questions into "objective" and "subjective" to solve this. They are testing "objective knowledge" using simple science quizzes with questions that are indeed pretty much factual and not under any dispute. I downloaded the questions to check that. For example: "All radioactivity is man-made: true/false", "Venus is the closest planet to the sun" etc. They call this objective knowledge and it seems mostly sound.
There was only one question I noticed where they appear to be asking for agreement with an (incorrect) consensus. They included in their "objective knowledge":
COVID-19 is transmitted mainly via small respiratory droplets through sneezing, coughing, or when people interact in close proximity. True
But this isn't right (in my opinion). Although they made the question ambiguous by adding the word "mainly", the data seems inconsistent with SARS-CoV-2 mostly spreading this way, and over time opinion shifted towards a much greater emphasis on aerosols that may be created even by people who are apparently asymptomatic. Nor was this unknowable at the time: SARS-1 apparently could spread through drainpipes in apartment blocks, so if SARS-2 is indeed similar to SARS-1 it makes sense it could spread in similar ways.
At any rate there are much bigger problems with the study than how they determine people's levels of scientific knowledge. The whole study is based on the idea that a bunch of highly controversial ideas are in reality "consensus", e.g. they consider that masks being highly effective is a consensus. But it certainly wasn't just before COVID, the consensus was the exact opposite. What is the point of even caring about a consensus of people who will change that consensus overnight without any new research findings?